This invention relates to the synthesis of mesoporous catalytic materials, generally known as molecular sieves.
Porous inorganic solids with molecular sieving properties, such as zeoiites, have been extensively used as heterogeneous catalysts and absorbants. This is because these materials have very large internal surface area, good thermal stability and, most importantly for catalytic applications, shape-selective and acidic properties. In many applications, particularly in the petroleum and petrochemical industries, molecular sieve zeolites totally dominate many established and most new processing technologies. However, most commercial zeolites are microporous with channel or cavity dimensions in the range of 5 to 14 .ANG.. This limits their application in processes dealing with larger molecules. Considerable effort has been devoted to develop a framework with pore diameters greater than 10 .ANG.. Recently, Mobil Oil Corporation has developed a family of mesoporous molecular sieve materials designated as M41S. This is a crystalline molecular sieve material with large pore diameters in the range of 15 to 100 .ANG.. The synthesis methods that are used are similar to those used in traditional zeolite synthesis except that large quaternary ammonium surfactant components were used. These new mesoporous products are typically prepared at temperatures in the range of 90.degree. to 150.degree. C. The production of such mesoporous catalysts is described, for instance, in Kresge et al U.S. Pat. No. 5,250,282, issued Oct. 5, 1993, Beck et al U.S. Pat. No. 5,108,725, issued Apr. 28, 1992 and Beck U.S. Pat. No. 5,057,296, issued Oct. 15, 1991.
It is believed that, like many thousand organic substances with an elongated, narrow molecular framework, the large organic ammonium surfactant molecules form a liquid crystal phase in its aqueous solution. Cationic surfactants are composed of groups of opposing solubility tendencies, typically an oil-soluble hydrocarbon chain and a water-soluble ionic group. Typically, the cationic surfactants have a hydrophilic head group, e.g. an ammonium group, with a positive charge and a long hydrophobic hydrocarbon chain or tail group. It is the hydrophilic portion of the molecule that enables the surfactant molecules to be miscible with water. However, at a given condition, the critical micelle concentration or "CMC" is relatively small. Therefore, as the concentration of the surfactant exceeds its CMC, the surfactant molecules tend to form miceiles. A minimum energy results when the surfactant molecules arrange themselves in such a way that there is a minimum contact between their hydrocarbon tails and surrounding water molecules. Thus, for cationic surfactants in water, micelles of different shapes may be formed. The hydrophilic heads of the surfactant molecules contact surrounding water molecules and the hydrocarbon chains or tails are hidden inside. Thus, as the surfactant concentration in the acueous solution exceeds its CMC, the cationic surfactant molecules form a liquid crystal phase. Such liquid crystal phase serves as a template as well as a catalyst for the formation of a regular aluminosilicate structure. When an as-synthesized product is calcined at high temperature, the surfactant molecules are decomposed and escape from the crystalline structure, creating the desired highly porous silica alumina molecular sieve framework.
Liquid crystals are materials which exhibit aspects of both the crystalline solid and the amorphous liquid state. They resemble liquids in their ability to flow, and solids in the degree of order within their structure. In many systems, this order is established spontaneously. In other cases, it can be brought about, or controlled, by electric, magnetic or hydrodynamic fields.
It is a primary object of the present invention to provide an improved process for producing mesoporous catalytic materials.